1. Field of the Invention
This invention relates to a projection-type optical apparatus used in the manufacture of semiconductor devices and, in particular, to a focal position correcting mechanism.
2. Related Background Art
Recently, a reduction-type projection printer based on the step and repeat system, called a "stepper", is generally being used in the lithography process in the production of semiconductor devices. It is an apparatus capable of projecting a minute pattern formed on a reticle (or mask) on a wafer and exposing it thereon with high resolution. As semiconductor devices become more integrated and more minute, the resolution limit of a stepper of this type is becoming higher from year to year. In a projection optical system having a large numeral aperture, the depth of focus is inevitably small. Further, any change in the environmental conditions around the projection optical system (the atmospheric pressure, temperature, humidity, etc.), the exposure condition (the condition of the illuminating light incident on the projection optical system), etc. causes the image formation characteristics (the focal position and the projecting magnification) of the projection optical system to vary. In particular, the variation in the focal position caused by a rise in temperature in the projection system due to its absorption of illuminating light, or to the temperature change in the projection optical system due to changes in the environmental temperature, occurs to a degree not to be neglected. At present, the deterioration in the image forming characteristics due to this variation in the focal position is a problem that should be solved.
In view of this, a system has been proposed in which the entire stepper is lodged in a clean chamber that is temperature controlled (e.g., within the range of 23.+-.0.1.degree. C.) and that maintains a certain degree of cleanliness. In this system, the projection optical system, which has an important influence on the image formation characteristics, is exclusively installed in a temperature control device which is separate from the chamber lodging the entire apparatus, with the temperature of the projection optical system being controlled by using a fluid which is under a more sophisticated temperature control, thereby efficiently avoiding deterioration in the image formation characteristics. Apart from this, a method of correcting the wafer position has been proposed in which the peripheral temperature around the projection optical system or the temperature of the projection optical system itself is measured so as to predict the variation in the image formation characteristics corresponding to the temperature change, thereby correcting the wafer position.
As stated above, in the above-mentioned conventional systems, the stepper is used in a chamber which is temperature controlled with an accuracy level of .+-.0.1.degree. C. However, in the case of a projection optical system having a large numerical aperture, even this level of accuracy in temperature control cannot be said to be enough to avoid deterioration in the image formation characteristics. Further, it should also be noted that a wafer stage, which is adapted to move with a wafer mounted thereon, is provided below the projection optical system. The displacement of this stage may cause a change in the conditions for the airconditioning of the projection optical system, thereby causing a temperature change in the projection optical system. In particular, the hardware for holding the lens elements, i.e., the so-called lens-barrel, is subject to expansion or contraction due to temperature changes. This constitutes one of the factors causing a variation in the focal position. It will be understood from the above that some other control technique as mentioned above is needed in addition to the temperature control by the chamber.
Of the above-mentioned prior-art techniques, however, the method in which the variation in the focal position is predicted from the temperature of the projection optical system, or the peripheral temperature, has the following problems: first, the variation in the focal position of the projection optical system is caused by a number of factors, including the temperature changes in the lens elements of the projection optical system, the temperature changes in the hardware or lens-barrel supporting the projection optical system, etc., which means it is not enough to measure the temperature of any one particular point to check the temperature of the entire system. Second, it should be noted that, when directly measuring the temperature of the projection optical system, the temperature rise in the projection optical system due to the absorption of the illuminating light by the system and the temperature change in the projection optical system due to changes in the environmental temperature are measured in a duplicated manner. Regarding the absorption of the illuminating light, the respective different absorption characteristics of the lens elements cause a temperature difference between the lens elements. As to the changes in the environmental temperature, the respective different heat capacities of the lens elements cause a temperature difference between the elements. Thus, these two categories of temperature change affect the temperature distribution in the projection optical system, i.e., the image formation characteristic thereof, in different ways, so that it is impossible to separate these two types of temperature changes from each other by a simple temperature measurement. It will accordingly be understood that the above-mentioned method is not practical for a projection optical system in which a strict focal position correction is required. In such a system, the most practical way is to adopt a system in which only the projection optical system is further temperature controlled so as to diminish the temperature change in the projection optical system.
Usually, in a stepper, the relative positional adjustment (focusing) between the plane in which the image of a reticle pattern can be formed with an optimum contrast (the optimum image forming plane) and the wafer surface is performed by adopting a correction method in which a grazing-incidence-type focus detection system (AF sensor), as disclosed in U.S. Pat. No. 4,650,983, is firmly attached to the periphery of the lens-barrel of the projection optical system in a mechanical manner so as to measure the deviation (the defocusing amount) of the wafer surface with respect to the optimum image forming plane. A focal position correcting mechanism, which drives the wafer in the direction of the optical axis of the projection optical system (the Z-axis direction), is offset in such a manner that the above defocusing amount measured attains a certain value (e.g., in the range of .+-.0.3 .mu.m), thereby correcting the wafer surface position with respect to the optimum image forming plane. Accordingly, even when the expansion/contraction of the lens barrel is avoided by the temperature control, the displacement in the Z-axis direction of the AF sensor due to temperature change, in other words, the relative displacement in the Z-axis direction between the hardware of the AF sensor and the projection optical system (the lens barrel), is allowed to arise as a focal position deviation, i.e., a defocusing amount. To avoid this, temperature stabilization has to be effected not only for the projection optical system but also for the AF sensor, with the result that the temperature control apparatus has to be a large-scaled one. Further, even when the projection optical system and the AF sensor are temperature stabilized, the accuracy in temperature control is at best in the range of .+-.0.05.degree. C., which cannot be called enough to avoid deterioration in the image formation characteristics. Furthermore, the difference in heat capacity between the projection optical system (in particular, the lens-barrel thereof) and the AF sensor (in particular, the supporting hardware thereof) leads to a difference in thermal absorption characteristic between them, so that the direction of expansion/contraction in one is reverse to that in the other, resulting in a still larger defocusing amount.